It seems that Nintendo Switch Online will not be required for free to play games. According to the eShop lisitings for most online games.
“To use online services you must have access to a wireless internet connection. Some online services may not be available in all countries. From the full launch of the Nintendo Switch Online membership service in 2018, online play requires a paid membership.”
Free to Play Games such as Fortnite, Paladins and Fallout Shelter lack this notice however, indicating that they will not require a subscription.
Nintendo has said that more information about Nintendo Switch Online is coming before launch, so hopefully this is cleared up soon.